Photovoltaic devices have recently been dramatically developed due to serious global warming and expectation of fossil fuel exhausting in the near future. The traditional photovoltaic devices, or solar cells, are based on silicon semiconductors such as single or multiple crystal silicon materials. However, these silicon materials are expensive, difficult for fabrication and in short supply in semiconductor industry. To overcome these problems, some substitutes such as thin film solar cells have been developed as the second generation of solar cells. At present, there are three main types of second generation thin film solar cells: amorphous silicon, CIGS and CdTe. In this thin film solar cell family, the CIGS solar cells possess the highest conversion efficiency that is as high as 20%, higher than 16% efficiency of the CdTe ones. In the periodic table of the elements, the elements of a CIGS absorber are located in Group IB-IIIA-VIA and the ones of CdTe absorber in Group IIB-VIA. These absorber materials all belong to multi-component p-type semiconductors. For such a semiconductor material, the distribution of different components, stoichiometry and crystal style may determine the quality of the materials.
Both of CIGS and CdTe solar cells contain a stack of absorber/buffer thin film layers to create an efficient photovoltaic heterojunction. A metal oxide window containing a highly resistive layer, which has a band gap to transmit the sunlight to the absorber/buffer interface, and a lowly resistive layer to minimize the resistive losses and provide electric contacts, is deposited onto the absorber/buffer surface. This kind of design significantly reduces the charge carrier recombination in the window layer and/or in the window/buffer interface because most of the charge carrier generation and separation are localized within the absorber layer. In general, CIGS solar cell is a typical case in Group IB-IIIA-VIA compound semiconductors comprising some of the Group IB (Cu, Ag, Au), Group IIIA (B, Al, Ga, In, Tl) and Group VIA (O, S, Se, Te, Po) elements of the periodic table, which are excellent absorber materials for thin film solar cells. In particular, compounds containing Cu, In, Ga, Se and S are generally referred to as CIGS(S), or Cu(In,Ga)(S,Se)2 or CuIn1-xGax (SySe1-y)n, where 0≦x≦1, 0≦y≦1 and n is approximately 2, and have already been applied in the solar cell structures that gave rise to conversion efficiencies approaching 20%. Here, Cu(In,Ga)(S,Se)2 means the whole family of compounds with Ga/(Ga+In) molar ratio varying from 0 to 1, and Se/(Se+S) molar ratio varying from 0 to 1. It should be noted that the molar ratios of Ga/(Ga+In) and Cu/(Ga+In) are very important factors to determine the compositions and the conversion efficiencies of the CIGS solar cells. In general, a good solar cell requires a ratio of Cu/(Ga+In) between 0.75 and 0.95, and Ga/(Ga+In) between 0.3 and 0.6. In comparison with CIGS, the composition of a CdTe solar cell is much simple. The content of Cd is usually close to 50% in the CdTe films. However, the Cd content may change after the deposition of a CdS layer and the subsequent annealing procedure. Close to the interface of the p-n-junction, for example, a CdSxTe1-x layer is formed with x usually not exceeding 0.06. However, x has a range changing from 0 to 1, which results in a compound from CdTe (x=0) to CdS (x=1).
Both CIGS and CdTe films has to be annealed to form a uniform stoichiometric compound. A CIGS film is usually annealed at a temperature between 350 and 600° C. in a typical two-stage fabrication procedure. For a CdTe solar cell, a CdS film may firstly be annealed in a superstrate configuration and a CdS/CdTe bilayer may be annealed in a substrate configuration. The importance of annealing is not only for the formation of a stoichiometric semiconductor material, but also for the determination of the crystal and boundary structures that may seriously affect the semiconductor properties. Therefore, the annealing should be well controlled in a carefully designed apparatus. For example, a CIGS film is usually going through a Rapid Thermal Processing (RTP) to approach a high temperature quickly at the beginning, followed by a reaction at the raised temperature in a super-pure inert, H2Se, H2S or Se atmosphere. After annealing, an n-type semiconductor buffer layer such as CdS, ZnS, or In2S3 should be deposited onto a CIGS semiconductor absorber. After then, transparent conductive oxide (TCO) materials, i.e., ZnO, SnO2, and ITO (indium-tin-oxide), should be deposited to form the solar cells.
The annealing process is sensitive to any impure species. For example, any residue oxygen or water inside a reactor may oxidize a CIGS absorber and destroy this semiconductor material. Therefore, an annealing reactor has to be totally isolated from the outside atmosphere. A vacuum apparatus should be helpful to clean the interior chambers at the beginning through some vacuum-inert gas cycles and remain a good sealing for the equipment during the reactions. However, most of high temperature reactors may not be designed as high vacuum apparatus. For these equipments, lots of inert gas has to be used to remain the annealing under a pure inert atmosphere. Especially for a roll-to-roll or reel-to-reel continuous process, more attention has to be paid to design a qualified reactor. Some high vacuum apparatus has recently been patented and assigned to SoloPower as a roll-to-roll reactor to anneal CIGS absorbers. For example, a few patent applications have been presented to provide methods and a high vacuum reactor to selenize and anneal CIGS absorbers in a roll-to-roll process, as shown in patent applications with publication numbers US2009/0183675A1, US2009/0148598A1 and US2010/0139557A1. With their designs, the reactor comprises a primary gap defined by a peripheral wall and an insert is placed within the gap to process the reaction of a continuously traveling workpiece. This insert processing gap is vacuum-tight and an inert or reactive gas can be introduced during the reaction under a certain temperature controlled by some heating elements surrounding the peripheral wall.
The main drawback for this reactor is that its integrated design requires a continuous piece of the peripheral wall. When the reaction needs time, the peripheral wall has to be designed very long or the travelling speed of the workpiece has to be very slow. The long peripheral wall may significantly increase the cost and the difficulty of fabrication, transportation and maintenance. In addition, the heating elements surrounding the body also significantly increase the temperature of the working environment. To solve these problems, the present invention provides a vacuum-tight reactor comprised with a series of modular sections. Moreover, the heating elements have been incorporated inside the reactor chamber that may be isolated with a vacuum chamber.